Is there a gene for every disease?

So far, we've examined mitochondrial and chromosomal diseases. For the final two -- single-gene and multifactorial diseases -- we'll focus in on what happens when mutations occur directly to our genetic code.

You could probably take a guess at what a single-gene disorder is just by its name. In single-gene disorders, a mutation occurs to one gene. This, in turn, means that the gene may not be able to provide the proper instructions for creating a much-needed protein, resulting in an illness. Cystic fibrosis, a disease affecting the mucus glands, is just one example of a single-gene disorder. Because of a genetic mutation that hinders regulation of fluid flow through cell membranes, people with cystic fibrosis have excessive mucus that gets in the way of normal body functions.

In the final type of genetic disorder we'll talk about, more than one gene gets in on the action. Multifactorial genetic disorders occur when more than one gene is affected by a genetic mutation. These mutations could be inherited from our parents or come about from our environment -- from exposure to chemicals, for example. Many of our most common ailments, such as diabetes, Alzheimer's disease and heart disease, have been labeled multifactorial genetic diseases.

Cancers are also multifactorial diseases. Different cancers are a result of multiple genetic mutations. Together, these mutations hide the critical blueprint needed to make proteins in charge of monitoring cell growth. As a result, cell growth gets out of control, leading to tumors.

Thanks to the mapping of our genetic code and other advances in research, scientists are continually making discoveries about the connection between our genes and diseases. However, it isn't always 100 percent clear just how much genetic mutations are at fault and how much we're driving the prevalence of certain illnesses ourselves. Case in point: obesity, which is caused by an imbalance between energy (calories) in and energy out.

Is obesity a direct result of our environment, or are genetics at play in this condition? Although we still have much to learn, it looks like both work together to contribute to obesity. Some scientists point to a theory that a genetic susceptibility to obesity comes from having ancestors who survived famine. Over time, the survivors passed on the genetic ability to "hold tight" to food energy. That's great news during a famine, but not so hot during times of plenty. Other theories point not just to this evolutionary change, but also to the possibility that genetics drive a person's excessive fat storage ability, diminished ability to turn fats into fuel, overeating habits and lack of physical activity [source: Centers for Disease Control and Prevention].

When it comes to obesity and the continued discoveries of how genetics are behind our illnesses, only time will tell. That said, in our fast-paced world, it may not be that much time. After all, the Human Genome Project, a massive effort of the National Institutes of Health and U.S. Department of Energy to map our complete genetic sequence, took just 13 years -- 1990 to 2003. Without a doubt, this research is on a fast-track schedule.